1. Field of the Invention
The present invention relates to a coupling device, such as a liquid tank joint, for discharging liquid in a liquid tank.
2. Description of Related Art
Liquids, such as high-purity semiconductor chemicals and general chemicals, are generally stored in a liquid tank, such as a polyethylene tank, in a production plant and are shipped with a lid attached to a charging/discharging mouth formed on the liquid tank. A known example of a method for discharging liquid contained in such a liquid tank is a siphon system in which gas, such as air, is introduced into a container so that the liquid is discharged outside the container due to the gas pressure thereof.
In this siphon system, after a lid attached to the mouth of the liquid tank is removed, a siphon serving as a liquid passage and a plug provided with a gas passage therein are attached to the mouth. By connecting, to the plug, a socket to which a tube for discharging the liquid to the outside of the liquid tank and a tube for introducing gas can be connected, a liquid passage for discharging the liquid and a gas passage for introducing the gas are formed. Such a liquid tank joint is disclosed in, for example, Japanese Unexamined Patent Application, Publication No. 2001-192099 and the Publication of Japanese Patent No. 3464232.
While a container having an inner-liner bag for containing liquid, such as a chemical solution, is being conveyed, gas is sometimes given off by the liquid and accumulates above the liquid. When the liquid in the container is let out, with the socket connected to the plug so that the gas accumulates, the gas in the inner-liner bag is let out and is splashing out from the container together with the liquid. To prevent the liquid from splashing out from the container due to the gas in the inner-liner bag, a socket configured to let out the liquid after releasing the inner pressure of the gas in the inner-liner bag is disclosed in Japanese Unexamined Patent Application, Publication No. 2002-114242.
However, the inventions disclosed in Japanese Unexamined Patent Application, Publication No. 2001-192099 and the Publication of Japanese Patent No. 3464232 have a problem in that assembling of the plugs is complicated because the number of components for forming a liquid passage and a gas passage in the plugs is large.
Furthermore, the configuration described in Japanese Unexamined Patent Application, Publication No. 2002-114242 has a problem in that external pressure supplied between the inner-liner bag and the container acts on the inner-liner bag to cause the liquid to splash out from the container during use.